Search and tracking of fast moving objects requires fast moving high spatial resolution radar beams. To achieve this goal, phased array antennas/radars operate with wide instantaneous bandwidths that requires true time delay radar beamforming implemented with variable delay lines. In addition, radar testing also requires variable delay lines. Radar systems operate over varied radar bands from S-band to X-band, plus these radar systems have a varying number of independently driven antenna elements and/or sub-arrays.
Other applications where variable optical delay lines are required include electronic warfare, buffering in data routers, RF signal processors/transversal filters, adaptive filters, and test and instrumentation. Photonics is a desirable technology for implementing delay lines as it is essentially RF bandwidth and time delay range insensitive.
Ideally, the RF radar delay line requires all the following attributes: (a) Modular design to upgrade time delay ranges up-to 20 ns for beam forming, (b) High Resolution time delay control in sub-picoseconds over the entire range, (c) Fast speed from milliseconds for radar testing to nanoseconds for advanced radar beam forming, (d) Greater than 60 dB RF inter-channel and intra-channel noise suppression, (e) Low <10 dB RF loss for complete module, and (f) Near smooth none granular time delay controls of >16 effective bits. Uses of fast optical switches in realizing digitally switched N-bit (N 6 bits) optical delay lines makes the delay module very lossy (e.g., 20 dB loss).
Today, the desired low loss fast speed of nanoseconds reset time, high resolution wide time delay dynamic range variable optical delay line for RF signal processing is yet to be disclosed. Hence, the opportunity to realize such a variable optical delay line is very significant for commercial and aerospace applications. The methods, systems, apparatus and devices of the present invention provide a variable optical delay line design that can realize these difficult delay line requirements.
A variable fiber optic delay line (VFODL) is a highly sought after component with applications ranging from microwave/millimeter wave analog photonic signal processing to digital optical communication systems based on packet switching. The ideal VFODL is able to efficiently and continuously generate time delays with high temporal resolution over any given long time delay range.
Over the years, efforts have been made to realize these variable fiber optic delay lines, particularly for microwave photonics applications where an RF signal riding on an optical carrier needs to be provided with a desirable delay. One way to efficiently generate many time delays over a long time delay range uses an N-bit switched binary architecture that employs 2×2 digital switches to select given binary paths connected in a serial cascade architecture. Here, based on the delay range required, free-space, solid-optic, and fiber-based delay paths have been deployed in both serial and parallel switched architectures using a variety of switching technologies such as liquid crystals.
Because of the digital switched nature of these variable fiber optic delay lines, time delay resolution is quantized to a discrete value and there is a tradeoff between resolution and number of binary switched stages. In effect, getting smaller resolutions across larger time delay ranges means adding more cascading, leading to higher losses and greater module complexity. Hence a dilemma exists to get both high resolution and long time delay range while keeping loss numbers down.
A more recent and attractive technology for generating analog controlled time delays involves the use of wavelength tuning and Fiber Bragg Gratings (FBGs) or dispersive optical fibers. Initially discrete fiber Bragg gratings positioned along specified fiber paths were used to produce discrete time delays based on the wavelength chosen. Later the concept was extended to use a chirped Fiber Bragg Gratings to generate near continuous time delay for a phased array control application, but over short time delay range due to the fabrication size limitations of Fiber Bragg Gratings and the specified laser small tuning range.
There are other similar works in use of FBGs for variable optical delays. Use of dispersive fibers and wavelength tuning to get optical delays has also been disclosed. To get more delay settings within an efficient structure, multi-wavelength fiber time delay processing was proposed using discrete FBGs delay segments within a serial optical switched structure (N. A. Riza and N. Madamopoulos, “Phased-array antenna, maximum-compression, reversible photonic beam former with ternary designs and multiple wavelengths”, in Applied Optics-IP, Vol. 36 (5), pp. 983-996 (1997).
More recently, efforts have replaced the standard high dispersion fiber in the R. Soref/R. Esman beamformer with a 6 times higher dispersion PCF as describe in Y. Jiang, et. al., “Dispersion enhanced photonic crystal fiber array for a true time delay structure X-band phased array antenna,” IEEE Photon. Tech. Lett., Vol. 17, pp. 187-189, 2005 that leads to 6 times reduction in fiber lengths, although at the cost of higher optical losses. Another wavelength sensitive design for a time delay beamformer uses a single fixed CFBG but variable optical filters splits the N delayed wavelengths by a factor of N to distribute to the N-antenna elements causing an inefficient beamformer design as N scales to larger numbers.
In addition, wavelength tuning in combination with wavelength division multiplexer devices was also proposed to realize variable fiber optic delay lines as described in N. A. Riza and S. Sumriddetchkajorn, “Micromechanics-based wavelength-sensitive photonic beam control architectures and applications,” in Applied Optics, Vol. 39, No. 6, pp. 919-932, February 2000. More specifically, the Arrayed Waveguide Grating (AWG) WDM device coupled with wavelength tuning has been extended to realize various VFODLs and RF filters as described in V Polo, B Vidal, J L Corral, J Marti, “Novel tunable photonic microwave filter based on laser arrays and N×N AWG-based delay lines,” IEEE Photonics Technology Letters (PTL), Vol. 15, 2003 and B. Vidal, D. Madrid. J L Corral, V. Polo, J. Marti, “Novel Dispersion-based Optical Delay Line using Arrayed Waveguide Grating for Antenna Beamforming Applications,” 28th European Optical Communication Conf. Proc. ECOC 2002, paper P3.17, Vol. 3, September 2002.
It is important to note that the V. Polo et. al. (2003 PTL paper) and B. Vidal ECOC 2002 papers use the periodicity of the AWG spectral response to feed multiple wavelengths into this specific arrayed waveguide grating WDM device with these wavelengths separated by the arrayed waveguide grating's free spectral range which limits time delay operations via dispersive fiber effects. Changing the wavelengths, but with the same separation given by the arrayed waveguide grating's free spectral range directs the light with many wavelengths into a different fiber port of the AWG device that has a different length of dispersive fiber; hence the time delay between the different wavelengths changes compared to light passing through another of the arrayed waveguide grating's fiber ports. In this case, long nanoseconds range time delays would need many km of dispersive fiber adding weight and temperature sensitivity to the variable fiber optic delay line module.
V. Polo et. al. (2003 PTL paper) and B. Vidal ECOC 2002 papers also suggest slight detuning of the multiple wavelengths, with wavelength gaps equal to the arrayed waveguide grating's free spectral range to fine tune their filter, but also point out that this fine tuning is highly limited (e.g., <3% of tuning range) to a very small wavelength shift (e.g., 50 GHz) as the AWG device's passband is practically limited.
What is needed is methods and systems for variable fiber optic delay lines that apply to any type of properly designed wavelength division multiplexing device and uses regular (non-dispersive) optical fiber for the long delays coupled with the wavelength division multiplexing devices and not the very long spools of dispersive fibers with the arrayed waveguide grating devices. In addition, the basic variable fiber optic delay lines of the present invention require wavelength division multiplexing device port specific bias delays for proper operations of the desired large time delay with the given high resolution signal production.